My first above-cited copending application describes a split-jaw lathe chuck having a chuck body rotatable about an axis and formed with a plurality of angularly spaced and radially extending inner guides and with a plurality of angularly spaced and radially extending outer guides. An inner jaw part wholly received within the chuck body is radially displaceable in each of the inner guides and an outer jaw part projecting axially from the chuck body is radially displaceable in each of the outer guides. An operating element is engaged with all of the inner jaw parts to jointly radially displace them. A coupling member is axially displaceable in each of the inner jaw parts between a coupling position in which the respective inner jaw part is locked for joint radial movement with the respective outer part and an axially offset decoupling position allowing relative radial displacement of the outer jaw parts and the respective inner jaw parts. These coupling members may be independently operable or jointly operable by means of a cam ring.
Such a split-jaw lathe chuck has proven extremely useful. The inner jaw parts can be displaceable only through a relatively limited radial stroke, so that is is possible for the operating member to achieve a considerable mechanical advantage for extremely tight clamping of a workpiece by the jaws. Workpieces of different sizes are accommodated by displacing the outer jaw parts relative to the inner jaw parts so as to position them in such a manner that the limited radial displacement of the inner jaw parts when coupled to the outer jaw parts allows these outer jaw parts to engage and clamp the workpiece, no matter what its size.
A disadvantage of this system is, however, that during setting of the outer jaw parts relative to the inner jaw parts it is possible for the inner jaw parts to move somewhat. It is also possible for the lathe to be started up when one of the outer jaw parts is not properly linked to its respective inner jaw part, so that the workpiece can slip out. What is more it is possible for a machine operator to set all of the outer jaws for a given workpiece size, only to find that the inner jaw parts were in their radial innermost position during such setting so that they will have to be moved to their outermost positions and all of the outer parts reset. It is noted that the operating member for the inner jaw parts is normally connected to some sort of safety arrangement that prevents the chuck from being rotated by the drive motor of the lathe until the jaws are tightly engaged with the workpiece. This can most easily be done by sensing the position of the operating member.